This invention is generally directed to a layered organic photoconductive device containing therein certain additives. More specifically, the present invention is directed to stabilized layered photoconductive devices comprised of photogenerating layers and transport layers, wherein the transporting material is rendered non-conductive. The additives of the present invention when added to the transport layer prevent a condition known as cycle down, which results in causing the photoconductive device to discharge on repetitive use resulting in images of low quality or no images whatsoever. Imaging members containing the additives of the present invention are useful for causing the formation of uniform electrostatic latent images, wherein the undesirable bulk trapping of holes in the transport layer does not occur.
There is described in the prior art several layered organic photoresponsive devices, including flexible devices, which have particular utility in high speed imaging machines. One such device contains a support layer, a photogenerating layer and a charge transport layer as described in U.S. Pat. No. 4,265,990. Another layered photoresponsive device is comprised of a substrate, overcoated with a hole injecting layer, which in turn is overcoated with a transport layer, a photogenerating layer, and finally a top coating of an organic insulating resin, as described, for example, in U.S. Pat. No. 4,251,612. As photogenerating layers for these devices, there can be selected various photoconductive materials, including trigonal selenium, selenium alloys, such as selenium arsenic, selenium arsenic tellurium, metal, and metal-free phthalocyanines including x-metal-free phthalocyanines, copper phthalocyanines; vanadyl phthalocyanine, and the like.
The charge transport layer is generally comprised of an organic charge transport molecule dissolved in a polymeric matrix material, this layer being substantially non-absorbing in the spectral region of intended use, for example, visible light, while also being active in that the injection of photogenerated holes from the charge photogenerating layer can be accomplished, and further the hole transport layer allows the efficient transport of positive charges to the surface of the transport layer. Examples of charge transport layers include various diamines, dispersed in resinous binders.
Illustrative examples of hole injecting layers include materials such as gold, graphite, and preferably carbon or graphite dispersed in a polymeric material such as a polyester.
While the above described photoresponsive devices function effectively, prolonged usage causes the conductivity of these devices to increase, this increase apparently caused by the photoxidative instability of the materials utilized in the charge transport layer. Photoresponsive devices containing charge transport layers comprised of, for example, diamines, are subjected to various oxidizing species during their use, including ultra violet radiation, which causes the diamines to become conductive, particularly, after prolonged charging, exposure, and erasing of the photoconductive member. Conversion of the transport molecules to a conductive state by oxidizing species causes the gradual build up of dark conductivity with useage and leads to a phenonemon known as cycle down. In order to prevent short and long term increases in undesirable dark decay, it is therefore important that the transport layer molecules be rendered insulating throughout numerous imaging cycles.
In accordance with the present invention, the transport layer molecules are converted to an insulating species by adding to the transport layer various stabilizers or additives, which transfer electrons to the conductive species contained in the transport layer material, and return this material to its non-conducting state. These stabilizers tend to prevent ultra violet degradation, and further are selected so as not to introduce hole traps of their own under cyclic use, since this can result in a cumulative trapping resulting in a residual build up or cycle up which is not desired. This trapping can be an isolated electronic state of the additive, or could result from the additive changing the character of the dispersion of the host molecule in the binder matrix.
The selection of diamine compositions as transporting layers for photoresponsive devices has been a recent development, thus, very little effort has been devoted to a study of these substances, particularly, as these studies apply to the production of latent electrostatic images over extended periods of time. Although it is not desired to be limited by theory, it is believed that the diamine charge transport molecules can be converted to an undesirable conducting state after a short period of time, thereby adversely affecting image quality. Accordingly, there is a need for compositions and processes whereby the charge transport diamine materials will remain in an insulating state throughout numerous imaging cycles.